An optical assembly in this context is understood to mean an assembly including a plurality of optical elements, for example mirrors or lens elements, which direct light or other radiation. In this case, the light or the radiation is focused or scattered, for example. Optical assemblies can be used for diverse purposes. Inter alia, they can be used as illumination systems in a lithography system. In such an illumination system, light is generated for later processing or use in a downstream portion of the lithography system.
Optical assemblies can also be used in the context of lithography systems in a subsequent step for producing integrated circuits or other micro- or nanostructured components. In this case, structured layers are applied to a substrate, such as a wafer, for example, wherein the layers, for structuring purposes, are firstly covered with a photoresist that is sensitive to radiation in a specific wavelength range. In particular, light or radiation in the deep ultraviolet (DUV: deep ultraviolet, VUV: very deep ultraviolet) or in the far, extreme ultraviolet spectral range (EUV: extreme ultraviolet) is used at present. The wafer coated with photoresist is exposed by an exposure apparatus, which can likewise be an optical assembly. In this case, a pattern of structures that is produced on a mask or a reticle is imaged onto the photoresist with the aid of a projection lens. Reflective optical units, for example, are used for this purpose. After the photoresist has been developed, the wafer is subjected to chemical processes, as a result of which the surface of the wafer is structured in accordance with the pattern on the mask. Further steps can follow until all layers have been applied to the wafer for forming the semiconductor structure.
The lithography system has a device for generating light or an illumination system and an optical imaging system, both of which can constitute a so-called optical assembly. In order to illuminate the wafer, firstly light is generated in the device for generating light, the light then being directed onto the wafer through or in the optical imaging system. Within the optical imaging system, also called projection assembly, optical units or optical elements are used in order to direct the light generated by the device for generating light onto the wafer. The optical units can be mirrors, for example. In order to ensure a precise imaging of the structures onto the wafer, firstly a precise alignment of the mirrors with respect to one another is desired. Secondly, it is desirable to be able to check and, if appropriate, correct the optical properties of the mirrors.
It is possible, for example, for imaging aberrations to arise as a result of the absorption of the projection light in the lens elements or mirrors or optical elements forming the optical system. Light-induced effects, such as nonuniform heating, can lead to local variations of optical properties of the optical elements. By way of example, increasing EUV powers provide for higher absorption loads on the optical elements and thus lead to larger temperature gradients. In order to be able to avoid or at least to be able to detect variations of optical properties brought about as a result, the optical elements can be monitored in a spatially and temporally resolved manner. A multiplicity of cable-based temperature sensors on the mirror rear side are usually used for this purpose. In this case, the spatial resolution correlates with the number of temperature sensors used. However, the cables used produce a dynamic short circuit between the mirror and a frame of the apparatus. Furthermore, the use of a large number of temperature sensors on or within the optical element places stringent demands on mirror manufacture and entails a certain risk of failure for the entire mirror, since the positions of the sensors are directly in proximity to the optically effective surface.
Furthermore, contaminants, in particular macroscopic contaminants, i.e. those having orders of magnitude of between a few μm and a few mm, on the mirror surface can alter the optical properties. Typical types of contamination are dust particles. Hitherto there has not been any possibility of ascertaining macroscopic contaminants on mirror surfaces individually for each mirror during operation. A decrease in the total transmission and in the optical power of the system heretofore has involved switching off the system in order to examine the mirror surfaces. Such contaminants can arise as a result of dust, outgassing from materials or the like.
Stray light in the projection system can likewise influence the optical properties of the mirrors, since it can lead to harmful light on the wafer, which can lead to a loss of contrast, for example. It is often desirable to acquire the temporal development of the stray light in order to be able to identify possible degradation effects at an early stage, in a similar manner to that in the case of the contamination changes.
The properties of the optical elements, in both illumination and imaging systems, can be further influenced by the position of the mirrors, which should therefore be checkable. Sensors are currently used for determining the absolute mirror position. On the basis of these mirror positions, the mirrors are brought from the end stop position, for example after the system has been switched off or after an initialization, approximately to their original position again. For this purpose, it is possible to use actuators, for example manipulators, which are present in the system in order to move the mirrors. However, the zero position of these sensors has an undesirable tendency toward drift, as a result of which the measurement of the position can become inaccurate over time.
Further properties of the optical elements concern the position and alignment within an optical assembly. At present there are no methods that can be used to directly determine the relative position of the optical surfaces of mirrors with respect to one another in an EUV projection system. It is particularly desirable to have knowledge of the position and the alignment of the optical mirror surfaces as early as in the construction of the lens during mounting. In this way, already at a very early point in time a first coarse optimization of the optical aberrations could be performed via corresponding mirror positionings. Hitherto, complex wavefront measurement techniques have been used for this purpose, the position and alignment of individual mirrors remaining unknown, however. The position and the alignment of the optical mirror surfaces are determined indirectly during mounting, for example, by CAA vectors (computer added alignment) of mirrors being measured prior to mounting and being taken into account computationally with locations or positions of manipulators at this point in time. Manipulators are used to alter the alignment and the position of the mirrors, the range in which the manipulators can act being limited. Since the manipulator travels are restricted, it would be desirable to be able to achieve a coarse adjustment as early as during mounting, in order to be able to use the restricted manipulator travels only for fine adjustment.
U.S. Pat. No. 8,339,577 B2 discloses an illumination system for a microlithographic assembly in which, inter alia, the alignment of mirror elements within a multi-mirror array can be determined. In this case, a luminous pattern is reflected by the multi-mirror array and acquired with the aid of a camera. One possibility for determining the position of mirrors with respect to one another is described in WO 2011/039036 A2, in which measurement sections which are defined between the mirrors and within which light passes are used to carry out an interferometric measurement.
However, measures for making it possible to determine various properties of the optical elements and the position of the optical elements within an optical assembly in a simple manner would be desirable.